I messed up my relationship with the one person I love more than anything on this planet. She is the one of the kindest and most generous people I've ever met. She treats me so good. I don't deserve her and I know she deserves better than me, but I want to become the man she deserves and I don't think I can do that anymore; she is done with me.
Last year I had a lapse in judgment -- a stupid moment, whatever you want to call it. I made a profile on some type of dating site. I had received an email from them in my spam folder, and I clicked on it for reasons not even I know. From there it just spiraled down hill. There were pictures on there and I got to looking at them. When it said I needed to register to look, I did. I created a profile just to look at other girls' pictures. I also put up a picture of myself and wrote a bio of sorts with some pretty shameful things. I'm disgusted with myself and now fiancés hates my guts. The moment I actually created it, I regretted it and tried to delete the profile and emails. I hide them from her because I was petrified that if she found out it would mean disaster for us. So I hid it, not so I could play games, I never went on it again after that first time. I thought I had deleted it. But I received an email from the site saying someone wanted to meet me, and my fiancé happened to be holding my phone. When she asked me about it my mind went into panic mode, and I immediately denied having any knowledge of it. Eventually, after I just made it worse by my denying it, I told her the truth. But by then it didn't do any good for us.
She has broken the engagement and has told me many time that she now hates me. I can't lose her. I know I should have thought of that before and should have told her right off the bat. But I was scared. I want the chance to be everything for her like she is for me. But I've hurt her so bad that she doesn't want to give me the chance, which I understand I don't deserve it, but she deserves so much better. It's literally killing me from the inside out.
I need any help I can get. What can I do to convince her that I can change given the chance?
While you tell me that you signed up for the dating site last year, you make it sound as if this email suddenly showed up months later. That would be difficult to imagine happening. It would make more sense that you continued to get emails, but didn't do anything about it. Spam happens all the time. Going to sites you should not have been on just increases the spam you receive. Sometimes the only way to deal with it is to change your email account.
The reason your former fiance doesn't trust you is because you lied about what happened and why it happened. It was the lies and not the temporary succumbing to temptation that likely upset her the most. What if it happens again? What if something worse happens? How will she ever be certain she can trust you? There is an important principle here, "He who is faithful in what is least is faithful also in much; and he who is unjust in what is least is unjust also in much" (Luke 16:10). How a person behaves in small matters reflects how he will behave in large matters.
Trust is one of those things that takes a long while to build, but can be destroyed in a moment. You can't gain trust by telling someone to trust you, nor can you prove you are trustworthy in advance.
She might change her mind and give you another chance, but for now you'll have to assume she will not. You need to move on with your life as if she won't be a part of it and begin the process of finding another woman to marry.